The present invention relates to a microstrip antenna having a planar patch antenna element.
An example of a related-art microstrip antenna will be described referring to FIGS. 4A to 5.
As shown in FIGS. 4A and 4B, a dielectric substrate 12 is placed on an upper face of a ground 10 formed of conductive metal, and a planar patch antenna element 14 is placed on an upper face of this dielectric substrate 12. Passing through the ground 10 and the dielectric substrate 12, a core conductor of a feeding cable 16 is electrically connected to the patch antenna element 14. A feeding point 18 of the patch antenna element 14 is at a position offset from a center of the patch antenna element 14.
For the directivity of the microstrip antenna having the above described structure, a high gain can be obtained in an upward direction, and a half power width can be narrowed, as shown in FIG. 5.
In a case where the above described microstrip antenna is arranged, in an electrically spaced manner, on a metallic conductor having a larger area than the ground 10, the directivity largely changes. An example of such an arrangement will be described referring to FIGS. 6A to 7.
As shown in FIGS. 6A and 6B, in a case where a metallic conductor 20 having a large area has been provided below the microstrip antenna, in an electrically spaced manner from the ground 10, the ground 10 and the metallic conductor 20 will be electrically coupled to each other, and a directivity having a decreased gain in an upward direction and a large half power width will be observed, as shown in FIG. 7. According to experiments, in a case where a distance g between the ground 10 and the metallic conductor 20 has an electric length of 1/10 to 11/12 of resonant frequency λ of the microstrip antenna, the half power width is largest, and in a case where the distance g is larger than ⅛ of the resonant frequency λ, almost no influence of the coupling appears. It is apparent that in a case where the distance g is zero, and the ground 10 and the metallic conductor 20 are electrically connected to each other, the metallic conductor 20 acts as the ground 10 having a large area, and the directivity is directed upwardly.
Moreover, the directivity is influenced not only by the metallic conductor 20 having such a shape as expanding around the entirety of the ground 10, as shown in FIGS. 6A and 6B. Japanese Patent Publication No. 2002-314323A discloses that the directivity is influenced, also in a case where the metallic conductor 20 has a rectangular shape of which short sides are shorter than the size of the ground 10, and long sides are longer than the size of the ground 10.
The microstrip antenna is employed, for example, as a GPS antenna and an antenna for ITS (Intelligent Transport System). In a case where this microstrip antenna is mounted on a vehicle, the microstrip antenna is usually arranged on a roof of a vehicle body or a dashboard made of metal plates. Consequently, the roof or the dashboard will act as the metallic conductor 20, and the directivity of the microstrip antenna is influenced. Under the circumstances, there is such an anxiety that a desired directivity cannot be obtained, depending on a manner of arrangement.